Feedback control systems such as employed in robotic actuators typically employ forward control elements for commanding a position or velocity of an object and feedback elements which produce signals that can be compared with the control systems to determine the direction and magnitude of required actuation of the object. In high precision systems, there is little margin for error in the processing of such feedback signals.
In order to achieve high resolution in positioning, it is necessary to employ fine resolution in the position sensing signals. However, when it is attempted to couple such fine resolution with high actuation speed of the object, a conflict exists because, particularly in a digital processing system, the cycle time for processing the data may exceed the time during which one or more complete cycles of the position sensing system are produced. Thus, if actuation speed is pushed too high, a position ambiguity of one or more complete cycles of the position sensing system may result.
Positioning systems frequently employ a home position sensor to provide a known point to which other positions are referenced. The home position sensor may be a mechanical, magnetic or electrooptic apparatus. In a high precision system, great care must be exercised in finding the exact home position. This process may be hindered by the fact that a home position sensor output may have numerous local maxima and minima and it is necessary to find the extreme one of these and identify it as the actual home position.
Even with a high resolution position sensing device, under conditions of extremely low velocity of the object, one or more complete cycles of position measurement time may ensue before the actual change in position has grown to a value sufficient to be resolved. As soon as the change in position is resolved, the value fed back is instantaneously changed by the one resolvable increment of position. This may tend to provide jerky control of the object since the feedback signal is provided only once per two or more periods of time during which position is measured.